As a power conversion apparatus for a direct current power supply with switching elements such as a power metal oxide semiconductor field effect transistor (power MOSFET), there are, for example, an inverter that converts direct current into alternating current and a DC-DC converter.
FIG. 17A shows a half bridge circuit used as a basis for one phase of an inverter. FIG. 17B is a time chart to explain operation of a power conversion apparatus 90 shown in FIG. 17A and shows an enlarged view of rise characteristics of a current Id and a voltage V2 when a switching element SW1 is turned ON.
The power conversion apparatus 90 surrounded by a dash-dot line shown in FIG. 17A is provided with the switching element SW1 on the high side and a switching element SW2 on the low side. By complementarily operating the switching element SW1 on the high side and the switching element SW2 on the low side, a direct current voltage E is converted to an alternating voltage, and power is supplied to an inductive load L.
A power MOSFET, an insulated gate bipolar transistor (IGBT), a super-junction (SJ) MOSFET, etc. are used for the switching elements SW1 and SW2 in FIG. 17A. Diodes D1 and D2 are respectively coupled in anti-parallel to the switching elements SW1 and SW2, and operated as freewheeling diodes when the inductive load L is driven. On the other hand, reverse recovery characteristics of the diodes D1 and D2 are generally poor. Therefore, as shown in FIG. 17B, a large spiked surge current due to a reverse recovery current (a current that flows in the opposite direction of the diode at the time of the reverse recovery) is generated on the current Id when the switching element SW1 is turned ON. Additionally, a surge voltage and a circuit resonance called ringing are induced to the voltage V2.
This phenomenon will be described as follows. In the half bridge circuit of FIG. 17A, during a dead time when the switching element SW1 on the high side and the switching element SW2 on the low side are turned OFF simultaneously, a circulating current in the forward direction flows from the inductive load L to the diode D2 on the low side. In this state, when the switching element SW1 on the high side is turned ON, the load current switches to the current Id that flows through the switching element SW1. At this time, a voltage in the opposite direction is applied to the diode D2, and as shown by waveforms of the current Id and voltage V2 in FIG. 17B, a large reverse recovery current is superimposed to generate a current surge and a voltage surge. Further, even after minority carriers, which are main ingredients of excess carriers injected from a P-region inside the diode D2, disappear and the diode D2 is turned OFF, the ringing shown in FIG. 17B is generated to cause noise. This ringing is caused by an LC circuit resonance generated by a parasitic inductance Lp of a wiring pattern and wire harness shown by a dashed line in FIG. 17A and a device capacitance Cd between the drain and the source of the switching element SW2.
Conventional power conversion apparatuses for a direct current power supply with switching elements have used a snubber circuit to suppress a spiked surge current generated at the time of ON/OFF of the above switching elements and the ringing associated therewith. A power conversion apparatus provided with the snubber circuit is disclosed, for example, in JP-A-2007-43797 (Patent Document 1) and JP-A-2010-206109 (Patent Document 2).
FIG. 18 is a circuit diagram of a power conversion apparatus 91 in which an RC snubber N is added to the power conversion apparatus 90 of FIG. 17A.
In the power conversion apparatus 91 shown in FIG. 18, the RC snubber N surrounded by a dash-dot-dot line and including a resistor R and a capacitor C, which are serially coupled, is coupled in parallel to the switching element SW2. The RC snubber N is the most generally used snubber circuit conventionally, in which the resistor R and capacitor C are added to reduce a value Q (Q=(1/R)·√(L/C)) of the series resonance and to increase the attenuation and to suppress the ringing (circuit resonance).
FIG. 19 is a diagram showing noise spectrums of the current Id of the above power conversion apparatus 90 (without the RC snubber N) and power conversion apparatus 91 (with the RC snubber N) to schematically show relationship between the frequencies and amplitudes, in which the ringing (circuit resonance) of each current Id shown in FIG. 17B is treated with Fourier-transformation.
As shown in FIG. 19, with the RC snubber N, an amplitude peak (resonance peak) of the ringing (circuit resonance) shifts toward lower frequencies from a resonance frequency f1 to a resonance frequency fn, and a maximum amplitude (peak value) is suppressed.
To sufficiently suppress the ringing (circuit resonance) in the power conversion apparatus 91 of FIG. 18, a capacitance value needed for the capacitor C of the RC snubber N is about 4 to 10 times of a capacitance value of the device capacitance Cd. Thus, for example in a semiconductor power module described in Patent Document 1, a multilayer ceramic capacitor is generally attached externally to a semiconductor chip to form the RC snubber. However, it is desirable to form the RC snubber on a semiconductor chip for size reduction.
On the other hand, in the power conversion apparatus of Patent Document 2, a freewheeling diode in which unipolar operation is carried out with a small reverse recovery current is used, a capacitor of the RC snubber is formed in a different position from a region in a semiconductor substrate in which a depletion layer is formed by the freewheeling diode, and a convergence time of an oscillation of a current and voltage generated at the time of reverse recovery operation of the freewheeling diode is reduced.
The power conversion apparatus of Patent Document 2 is reduced in size by forming the RC snubber on the semiconductor chip, but needs a Schottky barrier diode and a diode such as a soft recovery diode that carries out specific unipolar operation to control lifetimes of minor carriers. These diodes are different from a general PN junction diode. Additionally, by using the diode that carries out unipolar operation, a capacitance value of the capacitor of the RC snubber is small to some degree. Realistically, a capacitance value of the capacitor that can be actually formed on the semiconductor chip is limited to about one time of the device capacitance Cd. This capacitance value is insufficient to suppress noise.
When the spiked surge current and the ringing associated therewith shown in FIG. 17B and FIG. 19 are suppressed insufficiently, the spiked surge current and ringing variously affect devices around the power conversion apparatus as noise related to electromagnetic compatibility (EMC). Particularly, the spiked surge current and ringing greatly affect external devices which are coupled to the ground (GND) common to the power conversion apparatus and which share the GND wiring with the power conversion apparatus.